184 



and these form bubbles of the gas. Simultaneously the negative 

 ions are discharged at the positive electrode, and the atoms of 

 free chlorine unite to give molecules (Cla). 



Meanwhile, in the body of the solution, the departure of the 

 ions has disturbed the equilibrium HC1 ^ H+ + Cl~. The undis- 

 sociated part HC1 therefore continues to break up, attempting to 

 re-establish equilibrium, until the electrolysis is complete. 



By electrolysis of a solution of hydrochloric acid, therefore, 

 we obtain hydrogen and chlorine. With some electrolytes the 

 course of events is not so simple, secondary reactions taking place 

 at the electrodes. Thus when we pass a current through a solu- 

 tion of sodium chloride, we obtain chlorine at the positive elec- 

 trode, but hydrogen instead of sodium is liberated at the negative 

 electrode (see p. 139). In the same way, when we electrolyze a 

 solution of cupric sulphate, metallic copper is deposited on the 

 negative electrode, but the radical SC>4 cannot exist in the free 

 state, and reacts with the water present to liberate oxygen, ac- 

 cording to the equation: 



2S0 4 + 2H 2 -> 2H 2 S0 4 + O 2 . 



Even when we do not actually isolate the free radicals of an 

 electrolyte by electrolysis, however, we can show that they have 

 migrated with the current in the usual way by the fact that they 

 collect around the electrodes. Thus, in the electrolysis of so- 

 dium chloride, the solution around the negative electrode becomes 

 alkaline, owing to accumulation of sodium hydroxide. Similarly, 

 in the electrolysis of copper sulphate, the solution around the 

 positive electrode becomes add, owing to accumulation of sulphuric 

 acid. 



Jons and Displacement. When a metal acts upon a dilute 

 acid, and hydrogen is liberated, it is the ions alone that are di- 

 rectly concerned in the mechanism of the action. Thus the 

 equation for the action of zinc on hydrochloric acid : Zn -f 2HC1 



